The monitoring of high voltage, oil-filled electrical apparatus, such as transformers, reactors and capacitors, for the occurrence of acoustic events therein, is very desirable from a quality assurance standpoint. Such events could result from partial electrical discharges, magnetic domain switching (Barkhausen effect), high frequency mechanical vibrations, transformer tap changing, rainstorm or thunderstorm effects, and the like. With respect to partial electrical discharges, such discharges can degrade the insulation within the high voltage electrical apparatus causing the eventual breakdown of same. Thus, it is desirable to provide a device for detecting partial electrical discharges before such discharges increase to a level which may cause extensive damage to the insulation within the high voltage apparatus.
Various devices and methods are available for monitoring the occurrence of partial electrical discharges within oil-filled electrical transformers and high voltage capacitors. For example, a typical prior art method for detecting and locating partial electrical discharges within a transformer uses the electrical signal produced by the discharge. Another approach utilizes a signal responsive to the pressure pulse induced in the oil from the discharge to determine the intensity and approximate location of the partial electrical discharge within the transformer.
Rather than utilizing the electrical signal produced by or resulting from the partial electrical discharge, other detection devices monitor the acoustic events resulting from the partial electrical discharge. Such devices might use fiberglass rods which are located within the oil-filled transformer and which act as wave guides for transmitting the acoustic emissions resulting from the partial electrical discharges to sensors located exteriorly of the transformer. It has been found that such rods or wave guides have a tendency to impart their own acoustic properties on the signal being monitored thus degrading the performance of the detection device. Still another approach involves exteriorly affixing an acoustic sensor to the outer walls of the transformer. An inherent problem associated with this approach is that the resonances of the outer wall of the transformer degrade the performance of the device in a frequency range that is significant. In addition, spurious signals, such as those produced during a rainstorm or a thunderstorm, can degrade the acoustic signals detected by the device.
In view of the foregoing, it has become desirable to develop a device for detecting an acoustic event which is simple in construction and operation, rugged and durable so as to be capable of withstanding the aggressive environment that exists within an oil-filled apparatus, and which can accurately detect acoustic events from within the apparatus over an extended period of time.